We have employed several systems to investigate the fundamental problems of control of biological functions at the molecular level in animal cells. For this purpose we have been interested in virus replication and its natural inhibition since these can be studied intensively at the molecular level of organization. In the case of Adeno-associated virus we are investigating structure and function of what, in effect, is a single functioning gene. Early functions of an oncogenic agent, SV40 are also under study. These functions seem related to tumor induction. In the case of interferon we have begun to elucidate the mechanism of action of this extremely potent, natural biological defense mechanism. We have found that interferon is active against murine leukemia viruses. It may initiate its action by altering plasma membranes. The binding of interferon to cells and the subsequent activation of cell control mechanisms are under intense study at present. Finally, we are also looking at interferon's complex role in chronic virus infections.